If I am right, then the main work is in /wp-content/plugins/espresso-json-api/includes/resources/3.1/EspressoAPI_Registrations_Resource.class.php in the _checkin function.
There are no filters or hooks available.
Please note it is not recommended to modify a core file, and modifications are not covered under the support licence.
The support post ‘Hook for Ticketing Addon Attended’ is closed to new replies.
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